M. tuberculosis infects one third of the world's population. M. avium, a ubiquitous organism in the environment, is a major pathogen in AIDS patients. The interaction between mycobacteria and the macrophage is the critical step in the establishment of the infection. Mycobacteria are taken up by macrophages through phagocytosis and reside within phagosomes, where the bacteria survive and replicate. The principal defense mechanism of the macrophage against the mycobacteria is activation of the macrophage by IFN-gamma, resulting in increased mycobacterial killing. A more complete understanding of the resistance of the mycobacteria should help in developing new strategies for treatment and prevention of mycobacterial infections. Experimental studies in mice have shown a wide variation in virulence of different M. avium strains and the molecular basis for this difference in virulence as well as the relationship between virulence and the immune system is unknown. One possible mechanism is that virulent mycobacteria inhibit the ability of the infected macrophage to respond to IFN-g. Studies from the PI's laboratory have found that infection of mouse macrophages with a virulent strain of M. avium results in a dramatic reduction in gene expression induced by IFN-gamma. Analysis of the IFN-gamma signaling pathway, the JAK/STAT pathway, showed that mycobacterial infection inhibits IFN-gamma induced STAT1 activation and tyrosine phosphorylation. M. avium also inhibited tyrosine phosphorylation of JAK1, JAK2 and the IFN-gR alpha chain. This decrease in ability of the infected macrophages to respond to IFN-g was shown to coincide with a decrease in IFN-gamma receptor alpha and beta chain protein and mRNA expression. The PI has also observed that M. avium infection induces expression of SOCS-3, which has been shown to inhibit IFN-g signaling. These observations are the basis for the present application. The specific aims of the proposal are: 1) To determine the role of inhibition of IFN-g signaling in the virulence of M., tuberculosis and M. avium. 2) To determine the mechanism of IFN-gR inhibition in M. tuberculosis and M. avium infected macrophages. 3) To determine if receptor internalization by endocytosis and proteasomal/lysosomal degradation is responsible for the loss of IFN-g receptor in M. avium infected macrophages. 4) To determine the role of SOCS-3 in the inhibition of IFN-g signaling in M. avium infected macrophages.